1. From ER Diagrams to the Relational Model Rose-Hulman Institute of Technology Curt Clifton

2. Review – Entity Sets and Attributes  Entity set: collection of “things” in the DB  Attribute: property of an entity Soda name calories

3. Kinds of Attributes  Simple – single atomic value Soda name, calories  Composite – several sub-attributes PersonName(First,Middle,Last)  Multi-valued – set of values for one attribute Car color, Degrees earned (Somewhat rare, makes some searches harder)

4. Review – Keys  Let us tell entities apart  The key for an entity set is a subset of the attributes for that entity set, such that no two entities agree on all the attributes Soda name calories

5. Review – Relationships  Associate 2 or more entity sets  Constraints Maximums shown with numbers  Read like: a subject-verb-number-object Participation shown with double line  Read like: a subject-has to-verb… PersonSoda Most Likes N 1

6. ER Design Techniques  Avoid redundancy and don’t duplicate data  Don’t use entity set when attribute will do  Limit use of weak entity sets

7. N 1 Redundancy  Wastes space  Leads to inconsistency  For example: Soda name manf Manf name addr Made by

8. Failed Attempt At Fix Soda name manfManf addr  Delete Manf entity set  Add address to Soda

9. Successful Fix  Eliminate manf attribute from Soda  Use relationship to find manufacturer info. N 1 Soda name Manf name addr Made by

10. Don’t Use Unnecessary Entity Sets  Entity set should… Have at least one non-key attribute OR Be the “many” in a many-one or many-many relationship N 1 Soda name Manf name addr Made by

11. Bad Entity Set  Suppose we didn’t have manufacturer address N 1 Soda name Manf name Made by

12. Avoid Weak Entity Sets  Don’t try to be too clever  Can usually just add a unique ID  Government has done this for their databases: Social Security Numbers Vehicle Identification Numbers  But… Don’t trust uniqueness of IDs assigned by others

13. Why Use Weak Entity Sets At All?  Federated Databases, for example… All students in Indiana receiving state aid All players on FIFA soccer teams  One query sent to multiple DB  Still want a Conceptual DB Schema  But no global authority to assign unique IDs

14. The Relational Model  Originated as theoretical idea “A Relational Model of Data for Large Shared Data Banks”, E. F. Codd, Comm. of the ACM, 13(6), June 1970 http://www.acm.org/classics/nov95/s1p3.html  Revolutionized databases  Led to 1981 ACM Turing Award The “Nobel Prize of computing”

15. Relations  (Semi-) Formally Tuple: an ordered list n-tuple: an ordered list of length n Relation: a set of n-tuples  Informally: Relation: a table with unique rows Rows = tuples; Columns = attributes; Values in column = domain  Database: a collection of relations

16. Some Other Terms  Relation schema Describes a relation RelationName (AttrName1, AttrName2,…) Or RelationName (AttrName1:type, …)  Database schema Set of all the relation schema for the DB’s relations

17. Why is the Relational Model Dominant?  Very simple – just one data structure  Matches a “list the items” mentality  Easy to manipulate tables with UI  Forms basic foundation for SQL Relational model based on sets SQL based on bags (a.k.a., multi-sets)

18. From ER Diagrams to Relations  Entity sets become relations Columns are attributes of entity set  Relationships also become relations Columns are keys of participating entity sets

19. Example: Basic Entity Sets Soda name calories Customer name addr

20. Example: Basic Relationship Soda name calories Likes Most Likes Customer name addr Buddy With 1 Naming buddy Named buddy since

21. Simplifying!  Can avoid relations for many-one relationships  Just add key of the one to the relation of the many Soda Most Likes Customer 1 name addr name calories

22. Over Simplifying!  What happens if we try to eliminate relation for a many-many relationship?  Consider treating Likes as we did Most Likes Redundancy Data loss

23. Weak Entity Sets  Need enough columns to make rows unique!  So… All attributes of weak entity set + Key from supporting relationship Player Team Plays on name number 1 city

24. Entity Sets with Subclasses  Use nulls, or  Use multiple relations “ER Style”  How should we choose which to use? Soda namecalories Diet Soda sweetener isa